Britton doesn't like the term "killer app," because it gets misapplied and overused, and I agree. But remember the first time the term really was used? For anyone around in the early 1980s, it was the spreadsheet. Lotus 1-2-3, and before that, MBA. (Excel was a latecomer that gobbled up the market.)
Let's face it, companies would have never invested in PCs to help people write better letters -- we had our Wang word processors for that. It was the spreadsheet that made PCs useful to businesses, the rest (WP, graphics, etc.) was gravy.
That's why the latest announcement of StrikeIron seems intriguing. Essentially, for $99 a year, you can position your Excel spreadsheet as a client to various Web services. Application Development Journal's Mike Gunderloy provides a brief product review here. Maybe not a "killer app," but something that makes Web services real to the masses. Just like spreadsheets themselves made computers a tool of the masses.